Microleakage of Class V Methacrylate and Silorane-based Composites and Nano-ionomer Restorations in Fluorosed Teeth.

STATEMENT OF THE PROBLEM
Enamel and dentin marginal sealing ability of the new adhesive materials could play an important role in successful restoration on fluorosed teeth.


PURPOSE
The aim of this in vitro study was to evaluate the marginal microleakage of low-shrinkage silorane-based composite, nano-ionomer, and methacrylate-based composite through self-etching approach or with enamel acid etching.


MATERIALS AND METHOD
Seventy-two extracted human molars with moderate fluorosed (according to Thylstrup and Fejerskov index, TFI= 4-6) were randomly divided into six groups (n=12). Class V cavities were prepared on the buccal surface at the cementoenamel junction and restored with Clearfil SE Bond/Clearfil AP-X (methacrylate composite), Silorane Adhesive System/Filtek P90 , and nano primer/nano-ionomer according to the manufacturer's instructions (self-etching approach) or with additional selective enamel acid etching before primer application for each adhesive. After water storage and thermocycling, microleakages of the samples were assessed using dye-penetration technique at the enamel and dentin margins. Data were analyzed using non-parametric tests (α = 0.05).


RESULTS
There was a significant difference among the six groups at the enamel margin (p= 0.001), but not at the dentin margin (p= 0.7). For all the three adhesive materials, additional enamel etching resulted in significantly reduced microleakage at the enamel margin (p< 0.05).


CONCLUSION
Methacrylate- and silorane-based composites and nano-ionomer revealed a similar and good performance in terms of dentin marginal sealing, but not at the enamel margin. The additional selective enamel etching might improve enamel sealing for the three materials.


Introduction
Although role of fluoride in caries prevention is wellestablished, fluorosis is a side effect of its excessive intake. [1][2] Dental fluorosis is a kind of tooth malformation due to systemic overexposure to fluoride during tooth development. Drinking water containing high levels of fluoride, fluoride-containing supplements and die-tary products can be sources of fluoride. [1][2] The relatively high prevalence of fluorosis has been reported in different regions of Iran such as 100% in Makoo, 67% in Larestan and Bandar Lengeh and 67-80% in Dayer. [3][4] Fluorotic enamel reveals two layers: an acid resistant surface layer (hypermineralized with fluorapat-ite) and a porous hypomineralized subsurface layer. 2 Fluorosis severity has been classified based on the Thylstrup and Fejerskov-index (score 0-9 for normal, mild, moderate, and severe fluorosis). The clinical appearance of fluorosis is correlated to the histopathologic changes in the enamel by this index. [5] It has been found that with increasing fluorosis severity, the more porous subsurface enamel extends toward the inner enamel. These histological changes might result in chipping away the rather brittle wellmineralized surface enamel. [6] The subsequent exposure of the subsurface layer to surface attrition may lead to dentin exposure. Covering the dentin with an efficient adhesive material is necessary. In contrast to the moderately fluorosed enamel which is caries resistant, mild and moderately fluorosed dentin is shown to be caries susceptible. [7] This may be a result of changed morphology of the moderately fluorosed dentin, exhibiting hypomineralized areas of interglobular dentin with unfused minerals. [8] On the other hand, adhesive bonding to the fluorosed enamel has been demonstrated to be problematic due to further resistance to acid dissolution of fluorapatite than hydroxyapatite. [5] Therefore, establishment of effective adhesive bonding to both fluorosed enamel and dentin is of major importance for a successful adhesive restoration in the fluorosed teeth.
Some authors have recommended that the hypermineralized layer be removed by grinding away the outer surface using a diamond bur. [9] However, in some clinical situations, unground fluorosed enamel might be involved in adhesive restorations such as resin composite placement over the cavity margins without bevel. [9][10] Preservation of caries resistant enamel margin could be beneficial for durable restoration of the fluorosed teeth. Few studies reported bond strength of some adhesives to unground fluorosed enamel and dentin. [9,[11][12] Only one study evaluated microleakage of Class V composite restoration on mid-buccal/lingual surfaces of fluorosed teeth and reported a higher leakage in self-etched teeth than total-etched ones. [13] Therefore, this study was designed to investigate dentin and enamel marginal sealing of Class V cavities restored by using low-shrinkage resin materials, nanoionomer (NI) and silorane-based composite compared with methacrylate-based composite associated to a two-step self-etch adhesive. Also, the effect of an additional acid etching of enamel margin along with the three adhesive materials on marginal sealing was evaluated in fluorosed teeth.

Materials and Method
In this experimental study, 72 caries-free extracted human molar teeth were collected from 20-35 year old patients, living in fluorosis endemic areas of Iran with moderate fluorosis (TFI= 4 to 6). 5 The teeth were cleaned and stored in a 0.1% thymol solution during the three months taken for accumulation of the teeth. Standard Class V cavities (5mm wide, 3 mm high, and 2 mm deep) with the gingival margin of 1mm below the cementoenamel junction were prepared on the buccal surface of the teeth using fissure diamond burs (Teezkavan; Tehran, Iran) in an air/water cooled high speed turbine.
The bur was replaced for every five preparations. The prepared teeth were randomly divided into six groups   The dye penetration extents were scored for both the enamel and dentin margins from 0-3 as follows: 0= no dye penetration; 1= penetration of dye along the cavity wall, but less than one half of the length; 2= penetra-tion of dye along the cavity wall, but short of the axial wall; 3= penetration of the dye to and along the axial wall. [14] The obtained results were statistically analyzed using Kruskal-Wallis and Mann-Whitney U nonparametric tests at p< 0.05 level of significance.

Results
Microleakage scores for the enamel and dentin margins of the six groups are presented in Table 1 There was no significant difference between groups 1 and 3, 1 and 5, 3 and 5, 2 and 4, 2 and 6, and 4 and 6 (p> 0.05) ( Table 2), revealing a similar leakage for the three adhesive materials in the case of no etching and when etching the enamel margin was performed.

Discussion
With increasing prevalence of fluorosis in many areas of the world and widespread use of resin materials for ad-  Nano-ionomer (NI) is a novel highly packed nanofilled resin-modified glass-ionomer that has been recently introduced to dental market. In addition to advantages of RMGI, NI showed improved mechanical strength, resistance to biomechanical degradation and lower polymerization shrinkage. [18][19][20] These two types of resin materials might be a desirable restoration for fluorosed teeth. Ermis et al. [10] recommended that a good two-step self-etch adhesive along with selective enamel acid etching could provide reliable bonding to fluorosed teeth. Waidyasekera et al. [12] reported that the two-step self-etch adhesive, SEB revealed a higher bonding performance to fluorosed dentin than etch-andrinse and one-step self-etch adhesives. The separate hydrophobic bonding resin could provide better dentinal sealing. [10] In the current study, all adhesive materials showed a similar slight microleakage the dentin margin. Silorane composite was used associated with a two-step self-etch adhesive similar to Clearfil SE Bond. Silorane Adhesive System consists of a hydrophilic ultra-mild self-etch (pH=2.7) primer and a hydrophobic bond which was separately applied and light-cured. The resin layer has demonstrated to maintain the normal dentin-adhesive interface sealed against the ingress of water. [21] The incompletely mineralized fluorosed dentin with inhomogeneous mineral distribution has more water permeability. [8,12,22]  On the other hand, higher acid susceptibility of fluorosed dentin [7] resulted in more aggressiveness of separate phosphoric acid etching on the dentin. [12] Therefore, in the current study, the dentin was not etched; the mild and ultra-mild self-etching primers of self-etch adhesives used might bond to fluorosed dentin better than etch-and-rinse adhesives do.
NI bonds to the tooth structures using a selfetching nano primer (pH=3). This light-cured primer contains a monomer and a photoinitiator that may create a resin covering on the primer dentin similar to those of mild one-step self-etch adhesive. [24] It seems that wa- The specific tooth type and the age group used in this study minimized the effect of these factors on the fluoride content and consequent enamel adhesion. [11,30] The used teeth classified as TFI score 4-6 (moderate fluorosis) exhibited chalky white appearance and distinct pitting area on the enamel surface. In higher TFI scores, considerable parts of the surface enamel are lost.
[5] Moreover, the fluoride level of the surface enamel in fluorosed teeth with TFI=7-8 was reported to be similar to that in the teeth with TFI=5-6. [31] Further studies are required to evaluate the interaction of new adhesive materials with tooth structures involved in different severity of fluorosis.

Conclusion
According to our results, it can be concluded that the three adhesive materials based on self-etching approach, nano-primer/NI, Silorane Adhesive/Silorane composite and SEB/methacrylate composite revealed a good performance at the dentin margin but a poor performance at the enamel margin in terms of marginal sealing. Selective enamel acid etching improved the enamel sealing ability of the three adhesive materials in the fluorosed teeth.